The present invention relates to circuits for comparing amplitude modulated signals and more particularly circuits for comparing signals from video tape recorder heads.
Some video tape recorders (VTR) comprise four heads, two for a "normal duration" mode referenced S1 and S2 which serve both for recording and reading, and two for a "long duration" mode referenced L1 and L2 which also serve both for recording and reading. Generally, the duration of a recording or reading operation in the latter mode is twice the "normal duration" mode.
The four heads are integral with a circular drum 1 schematically shown in the top view of FIG. 1. The two heads S1 and S2 are diametrical as well as heads L1 and L2. Moreover, heads L1 and L2 are shifted by an angle .alpha. generally low, of about one thousandth radian, with respect to heads S2 and S1, respectively.
The VTR heads record or read data (picture and sound) on a magnetic tape 2 shown on FIGS. 1 and 2. Data are contained in rectilinear tracks 3 which exhibit a small angle .beta. with respect to the longitudinal axis of the tape. Tracks 3 are arranged side by side.
In operation, the drum rotates around its axis and the tape advances while being pressed against the drum over more than 180.degree. (FIG. 1). The heads are actuated at a relative speed with respect to the tape. In the "normal duration" mode, each of heads S1 and S2, one after the other, interacts with a single track. In the "long duration" mode, each of heads L1 and L2 interacts with a single track one after the other.
On the tape portions not pressed against the drum, referenced B1 and B2 in FIG. 1, the longitudinal axis of the tape is perpendicular to the drum axis. On portion B3 of the tape pressed against the drum, the longitudinal axis of the tape at each point presents with the drum axis an angle slightly different from .pi./2 so that, for a relative head speed adapted with respect to the tape, the heads follow a path T1, shown in FIG. 2, parallel to the longitudinal axis of the tracks.
In addition, such four-heads VTRs can generally operate in an additional mode, called "search" mode, wherein pictures are presented at an accelerated speed for rapidly looking for specific sequences. In such a mode, the tape advance speed is increased and the path of the heads on the tape is no longer parallel to the longitudinal axis of the tracks. Such a path T2 of a "search" mode head is shown in dotted lines in FIG. 2. In this event, a head advances, on the one hand, inside tracks and, on the other hand, on areas separating tracks. The passage of the heads on the separation areas impairs the picture quality. Indeed, in "normal duration" mode or in "long duration" mode, the signal obtained from the tracks during reading is a high frequency signal (100 kHz to 10 MHz) with a constant amplitude. In "search" mode, the amplitude of the high frequency signal will decrease each time the head cuts a separation area between tracks. Thus, the resulting signal will be a high frequency amplitude modulated signal. The amplitude decreases at the areas separating tracks correspond to noise bars on pictures.
Then, it has been devised, in the prior art, for eliminating those noise bars, a method wherein one no longer uses, in "search" mode, only the S-type heads ("normal duration" mode) or only the L-type heads ("long duration" mode). Since the picture degradation corresponds to signal decreases, a circuit has been provided for comparing the amplitude of the signals associated with the head S with the amplitude of the signals associated with the head L of a couple of adjacent heads on the drum (couples (S1, L2) or (S2, L1)) and for selecting the head for which the amplitude is the highest. Moreover, in video tape recorders implementing such an approach, the signals from heads S and heads L are phase shifted by a value very close to .pi.. This can be obtained by adjusting the advance speed of the tape. Performances are thus optimized, a decreasing signal being replaced by an increasing signal, and so on.
Such a circuit is shown in FIG. 3. It comprises two identical legs 21 and 22, leg 21 receiving at its input 23 signals from a "normal duration" head S and leg 22 receiving at its input 24 signals from a "long duration" head L.
An amplifier 25 is provided at the input of each leg. The output of this amplifier is connected to the input of a low-pass filter 26 used for eliminating noise peaks. A detector 27, usually formed by a diode D, a capacitor C1 and a resistor R, has its input connected to the output of the low-pass filter. This detector has a time constant R.times.C1 and supplies a signal corresponding to the upper envelope of the high frequency modulated signal. The output of the detector constitutes the leg output.
The output of each leg is connected to an input of a same comparator 28. According to the sign of the voltage difference between its two inputs, the comparator supplies a logic "1" (generally the supply volt 5 V) or a logic "0" (generally 0 V), which determines the selection of the head associated with the maximal amplitude.
However, the circuit of FIG. 3 presents a limited sensitivity. The voltages of the input signals are liable to reach 600 mV under satisfactory conditions, and they may not exceed 60 mV in case of poor quality recording, for example. If it is desired to significantly amplify the low intensity signals, by choosing an amplifier gain equal to 15 for example, the signals at the amplifier output will reach 5 V in case of signals initially having a high intensity. This would cause the amplifier to operate in saturation mode. Thus, the gain must not be too high, and not exceed 7 or 8.
In addition, the detector must have a high time constant R.times.C1 to ensure proper processing, usually about 1 ms. The resistance is about 10 k.OMEGA. and capacitance C1 is about 100 nF. Resistor R is integrable but capacitor C1 is too large to be integrated; it should be lower than about 10-20 pF. For decreasing C1 to such a value, the order of magnitude of R should be 100 M.OMEGA., which is very high and would produce a very scattered integrated resistor. The circuit of FIG. 3 is therefore not completely integrable.
An object of the invention is to provide a circuit for comparing high frequency amplitude modulated signals with a high sensitivity.
Another object of the invention is to provide such a fully integrable comparison circuit.